Ocean warming with climate change is forcing marine organisms to shift
their distributions polewards and phenology. In warm tropical seas,
evolutionary adaptation by local species to warming will be crucial to
avoid predicted desertification and reduction in diversity. However,
little is known about the adaptation of phytoplankton in warm seas.
Across the ocean, diatomic microalgae are the main primary producers in
cold waters; they also contribute to tropical communities where they
play a necessary role in the biological pump. Here we show that four
species of diatoms isolated from the tropical Red Sea adapted to warming
conditions (30 °C) after 200–600 generations by using various thermal
strategies. Two of the warming adapted species increased their optimal
growth temperature (Topt) and maximum growth rate. The other two diatoms did not increase Topt
and growth, but shifted from specialist to generalist increasing their
maximum critical thermal limit. Our data show that tropical diatoms can
adapt to warming, although trade offs on photosynthetic efficiency, high
irradiance stress, and lower growth rate could alter their competitive
fitness. Our findings suggest that adaptive responses to warming among
phytoplankton could help to arrest the sharp decline in diversity
resulting from climate change that is predicted for tropical waters.